(1) Field of the Invention
The present invention relates to: a plasma density information measuring method in plasma used in a producing process of a thin film component, a particle beam source or an analysis apparatus; a probe for measuring the plasma density information used for measuring the plasma density information; and a plasma density information measuring apparatus; and more particularly, to a technique for easily measuring the plasma density information over the long term.
(2) Description of the Related Art
In recent years, the use of plasma is increased. In a producing process of a thin film component, using high-frequency plasma generated by high-frequency power (high-frequency electric power) in a range from a RF frequency band of about 10 MHz to a micro frequency band of 2.45 GHz, etching process or CVD (chemical-vapor deposition) are conducted. In such a plasma application technique, it is extremely important for conducting appropriate process to sufficiently grasp the information concerning plasma density which excellently shows the characteristics of generated plasma. In the case of typical plasma comprising monovalent positive ion and electron, the positive ion density and the electron density are substantially equal to each other due to the properties particular to plasma that electrically neutral state is maintained, the electron density is generally called as plasma density.
Conventionally, as a method for measuring the electron density in plasma, there is an electron beam irradiation type plasma vibration probe which was developed relatively recently, in addition to a Langmuir/probe method and a microwave interference measuring method.
The Langmuir/probe method is a method in which a metal probe is directly exposed in plasma in this state, direct current bias voltage, or direct current bias voltage on which high-frequency voltage is superposed is applied to the metal probe, and based on the current value flowing through at that time, electron density is measured.
The microwave interference measuring method is a method in which a chamber for generating plasma is provided with windows which are opposed to each other with plasma positioned therebetween, microwave (e.g., single color laser light) is radiated to the plasma through one of the windows, and the microwave ejected from the other window is detected, and electron density is obtained based on phase contrast between the radiated microwave and ejected microwave.
The electron beam irradiation type plasma vibration method is a method in which a hot filament is placed in a chamber, and based on frequency of plasma oscillations generated when electron beam is irradiated to the plasma from the hot filament, electron density is obtained.
However, when the Langmuir/probe method is used for reactive plasma, there is a problem that the measuring can not be continued for a long time (i.e., life time is short). This is because that stains comprising insulative films are adhered on a measured metal probe within a short time, the current value flowing through the metal probe is varied, and accurate measurement can not be continued soon. In order to remove the stains adhered on the metal probe surface, a method in which negative bias voltage is applied to the metal probe to carry out sputter-removing method using ion, and a method in which the metal probe is allow to glow to evaporate and remove the stains have been attempted, but the effect is poor, and the problem is not solved by these methods.
Further, the microwave interference measuring method has a problem that the measurement can not be conducted easily. This is because a large-scale and expensive apparatus and adjustment of microwave transmission path are necessary, the phase contrast between the radiated microwave and ejected microwave is small and thus, it is difficult to measure precisely. Further, in the case of the microwave interference measuring method, there are drawbacks that only the average density can be obtained, there is no spatial resolution.
Furthermore, in the case of the electron beam irradiation type plasma vibration probe method, in addition to anxiety of plasma atmosphere contamination due to tungsten which is evaporated from the hot filament, there is a problem of anxiety of interruption of measurement caused by break of hot filament. Especially in the case of plasma using oxygen or chlorofluorocarbons gas, the hot filament is easily cut or broken, and it is necessary to frequently exchange the filament, it can not be said that this is practical.
In view of the above circumstances, it is an object of the present invention to provide a plasma density information measuring method capable of easily measuring the plasma density information over the long term, a probe for measuring the plasma density information, and a plasma density information measuring apparatus.
To achieve the above object, the present invention provides the following structure.
That is, a plasma density information measuring method of the present invention comprises the steps of:
supplying high-frequency power to plasma;
measuring a physical amount indicative of reflection or absorption state of the high-frequency power by plasma load; and
obtaining a frequency at which strong high-frequency power absorption is caused due to plasma density, i.e., a plasma absorption frequency based on the measurement result of the physical amount.
In the case of the plasma density information measuring method of the present invention, the high-frequency power is supplied to plasma, the physical amount indicative of reflection or absorption state of the high-frequency power by plasma load is measured (for example, the reflection amount of the high-frequency power or impedance value of the plasma load is measured). Based on the measured result of the physical amount, plasma absorption frequency at which high-frequency power resonant strong absorption is generated due to plasma density is obtained. If the high-frequency power resonant strong absorption is caused, since the physical amount indicative of reflection or absorption state of high-frequency power by plasma load is largely varied, plasma absorption frequency can easily be obtained. Since the obtained plasma absorption frequency has constant correlation with the plasma density, this is useful plasma density information. In the present invention, high-frequency power, i.e., high-frequency electromagnetic wave is supplied to plasma and thus, even if stains comprising insulative films are adhered to the antenna which supplies high-frequency power, there is little influence, and the plasma absorption frequency can be measured accurately.
In this point, the present invention is superior to the conventional Langmuir probe method. Because in this method, electric current flowing when ion in plasma reaches a surface of a metal probe is detected and therefore, if insulative film is adhered to the metal probe, it is impossible to measure. Further, according to the present invention, since a hot filament is not used unlike the electron beam irradiation type plasma vibration probe method, there is no anxiety of breaking of filament, and it is possible to obtain the plasma density information over the long term.
In the method of the invention, it is preferable that the high-frequency power is supplied to plasma through a division wall. By interposing the dielectric division wall between the plasma side to be measured and the supplying side of the high-frequency power, a foreign object should not enter the plasma from the supplying side of the high-frequency power, and plasma can be maintained clean. Further, in the case of reactive plasma also, the high-frequency power supplying side is not damaged. Furthermore, even if stains such as insulative films are adhered to the surface of the dielectric division wall, there is no change in the measuring system, it is possible to obtain the plasma density information for longer time.
In the present invention, for example, the physical amount indicative of reflection or absorption state of the high-frequency power by plasma load is measured by measuring an electric current amount of a high-frequency amplifier for supplying high-frequency power. Through the high-frequency amplifier for supplying high-frequency power, electric current corresponding to a degree of reflection or absorption of the high-frequency power by the plasma load flows. Therefore, it is possible to easily measure the physical amount indicative of reflection or absorption state of the high-frequency power by measuring this electric current.
In the present invention, for example, the reflection amount of high-frequency power is detected while sweeping high-frequency power frequency, and the plasma absorption frequency is obtained based on relationship between sweep-frequency and a detected result of the reflection amount of high-frequency power. That is, it is possible to easily obtain a frequency at which the reflection amount of the high-frequency power is largely reduced, as a frequency at which the high-frequency power resonant strong absorption is caused due to the plasma density, i.e., as a plasma absorption frequency.
In the present invention, a plasma surface wave resonance frequency is obtained as the plasma absorption frequency for example. The surface wave resonance frequency f is correctly corresponds to the electron density ne in plasma.
In the present invention, electron density in plasma to be measured is calculated in accordance with the obtained plasma surface wave resonance frequency. That is, the electron density ne in plasma is calculated in accordance with the surface wave resonance frequency f=xcfx89/2xcfx80 (wherein xcfx89 is surface wave resonance frequency). The electron density ne is substantially equivalent to the plasma density. The electron density ne can easily be calculated in accordance with the following equation (1):
ne=xcex5oxc2x7mexc2x7xcfx89p/e2xe2x80x83xe2x80x83(1) 
wherein xcfx89p: electron plasma angle frequency
[xcfx89p=xcfx89xc3x97{square root over ( )}(1+xcex5)]
xcex5:dielectric constant of dielectric division wall, xcex5o:vacuum dielectric constant
me: electron mass, e: electron amount
In the present invention, for example, Tonks-Dattner resonance frequency is obtained as the plasma absorption frequency. If the high-frequency power is radiated to the plasma, a plurality of absorption spectrum is observed in addition to the surface wave resonance. It is considered that this corresponds to so-called Tonks-Dattner resonance. That is, when electromagnetic wave is radiated from outside of cylindrical plasma and power absorbed by the plasma is measured, strong absorption is caused at plurality of frequencies around the electron plasma angle frequency xcfx89p. This phenomenon is cased as Tonks-Dattner resonance from the name of the detector. According to subsequent research, it is explained that the mechanism causing this resonance is that electron plasma wave transmitted in radial direction is excited by electromagnetic wave, and resonant absorption is caused when the excited electron plasma wave is reflected at the plasma end and standing wave is generated. Further, since there is relationship between the resonance frequency and the electron plasma angle frequency xcfx89p, if the plasma density is varied, the Tonks-Dattner resonance frequency is also varied. That is, the Tonks-Dattner resonance frequency provides plasma density information.
The present invention provides a probe used for measuring plasma density information, comprising:
a dielectric tube whose tip end is closed;
an antenna accommodated in the tube at its tip end side for radiating high-frequency power; and
a cable accommodated in the tube at its rear side and connected to the antenna for transmitting the high-frequency power.
When the plasma density information is measured using the probe used for measuring plasma density information of the invention, the probe is set such that the tip end of the tube is brought into contact with the plasma to be measured, the high-frequency power sent through the cable is supplied to the plasma from the antenna through the dielectric tube wall, and the reflection power of the high-frequency power required for measuring the plasma absorption frequency is received by the antenna, and taken out through the cable. Since the range where the high-frequency power from the antenna influences the plasma is not so wide, it is also possible to obtain a local plasma density information if the amount of the high-frequency power is adjusted. That is, if the plasma density information measuring probe of the invention is used, it is possible to easily prepare necessary state for measuring the plasma density information, and to obtain spatial resolution. Further, since the antenna is covered with the dielectric tube, plasma is not contaminated, and the antenna is not damaged by the plasma and thus, the lifetime is long.
In the probe used for measuring plasma density information of the invention, it is preferable that the antenna and the cable accommodated in the dielectric tube are capable of moving along a longitudinal direction of the tube such that a position of the antenna in the tube can be varied. In this example, the position of the antenna in the dielectric tube is changed along the longitudinal direction of the tube several times. And plasma absorption frequencies at the antenna positions are measured. Among the several absorption frequencies obtained by this measurement, the lowest frequency that is not varied even if the position of the antenna is changed is obtained as a surface wave resonance frequency.
In the probe used for measuring plasma density information of the invention, it is preferable that a conductor for preventing a leakage of ejected electromagnetic wave from the antenna is disposed at a position slightly back from the antenna such as to occlude a gap between the cable and an inner surface of the tube. With this structure, since the conductor disposed slightly back from the antenna prevents the electromagnetic wave power discharged from the antenna from leaking outside except plasma, measuring error due to the leakage of the high-frequency power is avoided.
In the probe used for measuring plasma density information of the invention, it is preferable that probe cooling means for forcibly cooling the probe is disposed. According to this example, since the probe is forcibly cooled by the probe cooling means, the measuring error by temperature rise of the tube or cable is avoided.
In the probe used for measuring plasma density information of the invention, it is preferable that the cable for transmitting high-frequency power comprises a conductor tube for a core wire and a shield, and an insulative ceramics material for filling a gap between the core wire and the conductor tube. According to this example, since the gap between the core wire and the conductor tube. According to this example is filled with the heat-resistant insulative ceramics material, the heat-resistance of the cable is enhanced.
In the probe used for measuring plasma density information of the invention, it is preferable that a surface of the dielectric tube is coated with metal such that a measuring area of the dielectric tube is not coated. According to this example, since the surface of the dielectric tube is coated with metal such that the measuring area of the dielectric tube is not coated, the local state of the measuring area that is not coated with metal is strongly reflected to the measured result, and the spatial resolution is enhanced.
In the probe used for measuring plasma density information of the invention, it is preferable that the antenna is extended closely along an inner surface of the dielectric tube. With this structure, since the high-frequency power irradiated from the antenna is effectively supplied to the plasma, the supply amount of the high-frequency power may be small, and the measuring precision is enhanced.
A plasma density information measuring apparatus of the present invention comprises:
sweep-frequency type high-frequency power supplying means for supplying high-frequency power to plasma while sweeping frequency;
reflection power amount detecting means for detecting a reflection amount of the high-frequency power; and
power reflection coefficient frequency characteristics obtaining means for obtaining a counter frequency variation of reflection coefficient of high-frequency power based on a sweep-frequency of the high-frequency power and the detected result of the reflection amount of high-frequency power.
According to the apparatus of the invention, it is possible to easily measure the plasma absorption frequency as the plasma density information.
In the apparatus of the invention, it is preferable that the apparatus further includes a dielectric division wall interposed between plasma and the sweep-frequency type high-frequency power supplying means. According to this structure, since the dielectric division wall interposed between plasma and the sweep-frequency type high-frequency power supplying means is provided, it is possible to maintain the plasma clean.
In the apparatus of the invention, it is preferable that the apparatus includes the above-described plasma density information measuring probe, and high-frequency power is supplied from the antenna in the tube to plasma using a tube wall of the dielectric tube as a division wall, a plurality of antennas are accommodated in the dielectric tube such that distances between a tip end of the tube and the antennas are different from one another, and the power reflection coefficient frequency characteristics obtaining means obtains a counter frequency variation of reflection coefficient of high-frequency power for each of the antennas, and a plasma absorption frequency appearing at the same frequency in the counter frequency variations is obtained as a plasma surface wave resonance frequency. With this structure, it is possible to easily measure the plasma density information, and to generate the spatial resolution. In addition, it is possible to easily obtain the plasma surface wave resonance frequency from the counter frequency variation of reflection coefficient of the high-frequency power from the antennas having different distances from the tip and of the tube.
In the apparatus of the invention, it is preferable that a plasma density information measuring probe is inserted in a chamber which generates plasma for forward and backward movement, and the probe is moved such that a tip end of the probe is pulled backward from a measuring position in the chamber to a retreat position in the vicinity of a wall surface of the chamber when measurement is not carried out. With this structure, since the probe is moved such that a tip end of the probe is pulled backward from a measuring position in the chamber to a retreat position in the vicinity of a wall surface of the chamber when measurement is not carried out, even if the plasma allows stains to adhered to the surface of the probe, it is possible to move the probe toward the plasma only when the measurement is carried out, to prevent the probe from being contaminated, and to keep using the probe for a long time.
In the apparatus of the invention, it is preferable that protecting means for blocking excessive plasma generating high-frequency power which enters the antenna in the probe is provided behind the plasma density information measuring probe. With this structure, when excessive plasma generating high-frequency power enters the antenna in the probe, the protecting means provided behind prevent the excessive high-frequency power, thereby preventing the apparatus from being destroyed. Especially when the generated plasma disappears unexpectedly, there is an adverse possibility that the high-frequency power for generating the plasma is directly placed on the antenna, and the probe control section is destroyed. However, this adverse possibility is overcome by the protecting means.